The question whether nanoparticles tend to agglomerate in water or not is essential for both the correct analysis of particle size distributions and concentrations of size fractions, as well as for evaluating the possibilities to remove these particles from water, e.g. by filtration.
TiO₂ (titanium dioxide) NP are widely used in sunscreens and wall paints and CeO₂ (cerium dioxide) NP are applied as catalyst in the automotive industry, as polishing agent in the semiconductor industry or in coatings of optical glasses [2]. Polystyrene nanoparticles may emerge from the fragmentation of plastic litter.
To obtain a better understanding how these three types of NP behave in the presence of hardness builders, different pH values and natural organic matter, Serge Stoll and his co-workers suspended these NP in: ultrapure water, three bottled mineral waters, in water from the Lake Geneva and in drinking water produced from Lake Geneva water.
By analyzing the NP’s surface charge (via zeta potential measurement) and sizes (as hydrodynamic diameters via laser doppler anemometry and dynamic light scattering as well as via scanning electron microscopy) the authors showed that both water hardness and negatively charged natural organic matter are playing important roles in connection with the surface charge of the NP. Negatively charged TiO₂ and CeO₂ NP tend to agglomerate at environmental pH if special cations (hardness) are present. The adsorption of natural organic matter to positively charged PS NP is considered as key process in the destabilization of the PS NP.
References:
[1] L. Ramirez, S. Ramseier Gentile, S. Zimmermann, S. Stoll: behaviour of TiO2 and CeO2 Nanoparticles and Polystyrene Nanoparticles in Bottled Mineral, Drinking and Lake Geneva Waters. Impact of Water hardness and Natural Organic Matter on Nanoparticle Surface Properties and Aggregation. Water 2019, 11, 721, doi: 10.3390/w11040721
[2] www.nanopartikel.info